1. Field of the Invention
The present invention relates to refrigerators. More particularly, the field of the invention is that of refrigerator control units for maintaining the internal refrigerator temperature.
2. Prior Art
Refrigerator control units generally include a thermostat that activates the refrigerator compressor when the internal refrigerator temperature rises above a set point. The thermostat also de-activates the compressor when the temperature falls below a second set point which may be different from the first set point. Conventional thermostats include mechanical parts which respond to temperature changes and activate an electrical switch.
For example, one conventional mechanical thermostat device includes a sealed tube in communication with a diaphragm connected to an electrical contact. Liquid alcohol or another temperature responsive volatile fluid is disposed in the tube and expands or contracts according to the ambient temperature, thus moving the electrical contact of the diaphragm and opening or closing the switch. With the tube disposed in thermal contact with the cold box or refrigerated portion of the refrigerator, typically the refrigerator evaporator, the thermostat switches on and off according to the cold box temperature.
Several problems may occur with such a conventional mechanical thermostat device. One problem is that the physically moving portion of the thermostat may be subject to wear which degrades the responsiveness of the thermostat. Also, the tube may become physically damaged from ordinary transportation or use of the refrigerator and disrupt the hermetically sealed environment inside the tube. Further, the cost of conventional mechanical thermostat devices is greater than the remainder of the thermostatic circuitry.
Another conventional mechanical thermostat device includes a bimetal element, such as a plate or disc. The inclination of the bimetal element varies according to the temperature and acts as a self-actuating switch when connected to one electrical contact and disposed in proximity to another electrical contact. The bimetal element bends, varying its incline sufficient to touch the unconnected contact and complete the circuit.
This conventional mechanical arrangement has similar problems with wear as the capillary tube, and also may suffer from metal fatigue and corrosion. In addition, the bending action of the bimetal element may need calibration, for example bias applied by a spring mechanism, to operate effectively. Requiring calibration adds to the cost of manufacture of the refrigerator, and may also lead to further expenses if the bimetal element is disturbed and requires further calibration.
An additional conventional thermostatic device is a solid state electrical circuit including a temperature responsive resistance element and a microprocessor control. The temperature responsive resistance element is typically disposed in a bridge circuit and attached to the microprocessor so that the microprocessor may determine the temperature of the cold box of the refrigerator and activate or de-activate the compressor according to its programming.
This conventional solid state electrical circuit has advantages over mechanical thermostatic devices in regards to reliability and durability. The physical device which responds to the temperature is much less subject to damage from wear, corrosion, metal fatigue, or other physically degrading conditions. Also, the solid state thermostatic circuit does not require mechanical calibration or servicing, and its temperature responsiveness is unlikely to change after manufacture. However, providing a microprocessor greatly increases the cost of the refrigerator cold control. Particularly in applications which require only an On/Off control, a microprocessor based cold control system is prohibitively expensive.
Other prior art temperature controls are known which include a thermistor which varies its resistance according to its temperature. The thermistor is connected in a circuit with a switching device, such as an operational amplifier with a regulating transistor or alternatively a Schmitt trigger circuit constructed from discrete components. An operational amplifier compares the voltage drop across the thermistor with a predetermined voltage to turn on its regulating transistor and thereby determine the On/Off set point of the cold control. The Schmitt trigger circuit switches when the voltage drop across the thermistor becomes sufficiently high to cause conduction which then activates a power relay to the refrigerator compressor.
For portable refrigerators which are driven by a direct current power source, the predetermined voltage is provided by a battery. However, the voltage supplied by the battery is also subject to ripple voltages caused by feedback from the inverter. These ripple voltages may falsely switch the switching device when the temperature of the thermistor has not significantly changed. Further, as the battery is drained, the ripple voltage fluctuations become more acute. Having the cold control fluctuate, and thereby successively turn the inverter on and off, subjects the inverter to undesirable starting and stopping which adversely effects the compressor that is driven by the inverter.
For refrigerators which are powered by direct current and those which are powered by alternating current, the difference between the On and Off set points is determined by the hysteresis of the switching device. A switching device with a hysteresis which is too small may be wasteful of energy because of the frequent cycling of the compressor. A switching device with a hysteresis which is too high may result in excessive cold box temperature variation. However, the difference between the On and Off set points is limited to a small percentage of the switching voltage of the switching device. Further, the hysteresis is determined by the inherent characteristics of the discrete comparator or Schmitt trigger circuit, and thus cannot easily be changed. Another discrete switching device may be substituted, but such a substitution may be difficult to accomplish and may also require additional changes in the associated circuitry. For portable refrigerators, the ripple voltages may overcome the inherent hysteresis of the switching device and cause the undesirable on and off cycling.
What is needed is a thermostatic device which is not as subject to wear as conventional mechanical thermostats.
Also needed is a thermostatic device which is less susceptible to physical degradation.
A further need is for a less costly thermostatic device.
Another need is for a thermostatic device which avoids successive activation and deactivation of the inverter and the compressor.
A still further need is for a thermostatic device which does not depend solely on the inherent hysteresis of the switching device to determine the On/Off set points.